132 N'. R. J miner .- 



Specimen No. 138, illustrates the extremely altered wall rock near" 

 the veins. In hand-sj^ecimen, it is a greenish coloured fine-grained 

 rock, greatly impregnated with pyrites, and veined by quartz 

 carrying a little stibnite and carbonate. None of the original 

 minerals of the diorite are recognisable. Microscopically chlorite 

 has entirely disappeared, and is replaced by massive mu,scovite, 

 giving a biaxial figure and having a fairly wide axial angle. 

 Streaks of carbonate have formed along the cleavage planes. Iron 

 pyrites, arsenical pyrites, and stibnite are all abundant in the 

 altered wall rock. The iron pyrites is often present in irregular 

 shaped grains, and when idiomorphic its form is usually the cube, 

 or a combination of the cube and octahedron, and more rarely, the- 

 pyritohedron. Arsenical pyrites is more abundant than the iron 

 pyrites close to the veins, and is readily distingushed from the 

 latter mineral by its silvery lustre in reflected light, and by its 

 idiomorphic outlines. Common forms are combinations of the unit 

 prism and the brachydome, giving wedge-shaped and hexagonal 

 sections. Cruciform twinning, which is so common in the arseno- 

 pyritei from the Diamond Creek Dyke, is only rarely seen in the 

 present case. Stibnite occurs singly in prismatic and acicular 

 crystals and as a network of these fibres. When massive, it is silver- 

 grey in reflected light. Some of this material, appearing metallic 

 in reflected light, is deep red in transmitted light. 2 It is most 

 common in the vein quartz, and in the quartz grains adjoining 

 the vein walL These three sulphides, although independently 

 crystallised, are undoubtedly essentially contemporaneous in origin. 

 Quartz still remains clear, but the grains are embayed, and partly 

 replaced by carbonates. The ilmenite has entirely disappeared, and 

 very little leucoxene remains anywhere in the section. Little 

 pyrites, either the iron variety or the arsenical type, is present 

 in the vein quartz, and the assumption is that the ilmenite and 

 the chlorite have supplied the necessary iron to form the pyrites 

 so abundant in the wall rock. 



Summary of the effects of the vein solutions : — In the absence of 

 a chemical analysis of the wall rock, no definite conclusions can be 

 drawn with regard to chemical migrations of the original consti- 



1 In the author's petrologlcal description of the Diamond Creek dyke, arseiiopyrite i.s mentioned' 

 as being possibly present. Further examination, however, shows it to be abundant near the vein 

 fissures. Vide, N. R. Junner, Proo. Koy. Soc. Victoria, vol. xxv.. p. 339, 1912. 



2 Professor MiKj^e was able to prove that stibnite showed straijfht extinction by examination 

 of very thin flakes of the mineral between crossed nicols in direct sunlight. Vide Neues. Jahrb. 

 Min., vol. i., p. 12, 1898. 



